5. GCSE level practise exam chemical calculation
questions on working out simple empirical formula from reacting masses
(NOT moles)
Type in formula without subscripts e.g. Fe3O4 even though it is correctly written as Fe3O4
(updated Mar 24th 2026)
If 54g of aluminium combines with oxygen to form 102g of aluminium oxide, what is the formula of aluminium oxide? (Relative atomic masses: Al = 27 and O = 16) [emp-1]
In an experiment, 56g of iron combined with exactly 64g of sulphur. What is the formula of the iron sulphide formed? (Relative atomic masses: Fe = 56 and S = 32) [emp-2]
In an experiment, 21g of lithium combined with exactly 14g of nitrogen. Work out the formula of lithium nitride. (Relative atomic masses: Li = 7 and N = 14) [emp-3]
If 168g of iron combined with oxygen to form 232g of an iron oxide, work out the formula of the iron oxide. (Relative atomic masses: Fe = 56 and O = 16) [emp-4]
If 28g of silicon combined with oxygen to form 60g of silicon oxide, work out the formula of silicon oxide. (Relative atomic masses: Si = 28 and O = 16) [emp-5]
In an experiment, heating 128g of copper combined with exactly 32g of sulphur. Work out the formula of the copper sulphide formed. (Relative atomic masses: Cu = 64 and S = 32) [emp-6]
In an experiment, excess chlorine was passed over 24g of heated magnesium metal until all the metal had reacted. If 95g of white magnesium chloride solid was formed, work out the formula of magnesium chloride. (Relative atomic masses: Mg = 24 and Cl = 35.5) [emp-7]
In an experiment, excess chlorine was passed over 27g of heated aluminium metal until all the metal had reacted. If 133.5g of white aluminium chloride was formed work out the formula of aluminium chloride. (Relative atomic masses: Al = 27 and Cl = 35.5) [emp-8]
In an experiment, excess chlorine was passed over 28g of heated silicon until all of it had reacted. If 170g of liquid silicon chloride was formed work out the formula of silicon chloride. (Relative atomic masses: Si = 28 and Cl = 35.5) [emp-9]
In an experiment, excess chlorine was passed over 31g of hot phosphorus until all of it had reacted. If 208.5g of white phosphorus chloride was formed work out the formula of phosphorus chloride. (Relative atomic masses: P = 31 and Cl = 35.5) [emp-10]
If 8g of sulphur combines with 12g of oxygen, what is the formula of the sulphur oxide formed? (Relative atomic masses: S = 32 and O = 16) [emp-11]
If 16g of sulphur combines with 103.5g of lead, what is the formula of the lead sulphide formed? (Relative atomic masses: Pb = 207 and S = 32) [emp-12]
If 40g of calcium combines with 160g of bromine, what is the formula of calcium bromide? (Relative atomic masses: Ca = 40 and Br = 80) [emp-13]
78g of potassium combines with 160g of bromine. What is the simplest formula of potassium bromide? (Relative atomic masses: K = 39 and Br = 80) [emp-14]
48g of magnesium combines with 320g of bromine, what is the formula of magnesium bromide? (Relative atomic masses: Mg = 24 and Br = 80) [emp-15]
160g of calcium combines with 64g of oxygen, what is the formula of calcium oxide? (Relative atomic masses: Ca = 40 and O = 16) [emp-16]
What is the formula of the phosphorus oxide formed when 62g of phosphorus combines with 80g of oxygen? (Relative atomic masses: P = 31 and O = 16) [emp-17]
72g of magnesium combines with 48g of oxygen, what is the formula of magnesium oxide? (Relative atomic masses: Mg = 24 and O = 16) [emp-18]
46g of sodium combined with 16g of oxygen. What is the formula of the sodium oxide formed? (Relative atomic masses: Na = 23 and O = 16) [emp-19]
78g of potassium combined with 32g of sulphur. What is the formula of potassium sulphide formed? (Relative atomic masses: K = 39 and S = 32) [emp-20]
56g of iron combined with 240g of bromine. What is the formula of the iron bromide formed? (Relative atomic masses: Fe = 56 and Br = 80) [emp-21]
If 20.9g of bismuth combines with 24.0g of bromine, what is the formula of the bismuth bromide formed? (Relative atomic masses: Bi = 209 and Br = 80) [emp-22]
If 54g of aluminium combines with 96g of sulphur, what is the formula of aluminium sulphide? (Relative atomic masses: Al = 27 and S = 32) [emp-23]
When 117g of potassium combines with 31g of phosphorus, what is the formula of the potassium phosphide formed? (Relative atomic masses: K = 39 and P = 31) [emp-24]
If 54g of aluminium combines with oxygen to form 102g of aluminium oxide, work out the formula of aluminium oxide. (Relative atomic masses: Al = 27 and O = 16) [emp-1]
Al2O3
Al2O
AlO2
AlO3
In an experiment, 56g of iron combined with exactly 64g of sulphur. Work out the formula of the iron sulphide formed. (Relative atomic masses: Fe = 56 and S = 32) [emp-2]
FeS
FeS2
Fe2S
FeS3
In an experiment, 21g of lithium combined with exactly 14g of nitrogen. Work out the formula of lithium nitride. (Relative atomic masses: Li = 7 and N = 14) [emp-3]
LiN2
Li2N
Li3N
LiN3
If 168g of iron combined with oxygen to form 232g of an iron oxide, work out the formula of the iron oxide. (Relative atomic masses: Fe = 56 and O = 16) [emp-4]
FeO
FeO2
Fe2O3
Fe3O4
If 28g of silicon combined with oxygen to form 60g of silicon oxide, work out the formula of silicon oxide. (Relative atomic masses: Si = 28 and O = 16) [emp-5]
SiO2
SiO
Si2O
SiO3
In an experiment, heating 128g of copper combined with exactly 32g of sulphur. Work out the formula of the copper sulphide formed. (Relative atomic masses: Cu = 64 and S = 32) [emp-6]
CuS
Cu2S
CuS2
CuS4
In an experiment, excess chlorine was passed over 24g of heated magnesium metal until all the metal had reacted. If 95g of white magnesium chloride solid was formed, work out the formula of magnesium chloride. (Relative atomic masses: Mg = 24 and Cl = 35.5) [emp-7]
MgCl
Mg2Cl
MgCl2
MgCl3
In an experiment, excess chlorine was passed over 27g of heated aluminium metal until all the metal had reacted. If 133.5g of white aluminium chloride was formed work out the formula of aluminium chloride. (Relative atomic masses: Al = 27 and Cl = 35.5) [emp-8]
Al3Cl
AlCl2
AlCl4
AlCl3
In an experiment, excess chlorine was passed over 28g of heated silicon until all of it had reacted. If 170g of liquid silicon chloride was formed work out the formula of silicon chloride. (Relative atomic masses: Si = 28 and Cl = 35.5) [emp-9]
SiCl4
SiCl2
Si2Cl
Si4Cl
In an experiment, excess chlorine was passed over 31g of hot phosphorus until all of it had reacted. If 208.5g of white phosphorus chloride was formed work out the formula of phosphorus chloride. (Relative atomic masses: P = 31 and Cl = 35.5) [emp-10]
PCl3
PCl5
PCl6
PCl7
If 8g of sulphur combines with 12g of oxygen, what is the formula of the sulphur oxide formed? (Relative atomic masses: S = 32 and O = 16) [emp-11]
SO
SO2
SO3
SO4
If 16g of sulphur combines with 103.5g of lead, what is the formula of the lead sulphide formed? (Relative atomic masses: S = 32 and Pb = 207) [emp-12]
Pb2S
PbS2
PbS6
PbS
If 40g of calcium combines with 160g of bromine, what is the formula of calcium bromide? (Relative atomic masses: Ca = 40 and Br = 80) [emp-13]
CaBr2
CaBr
CaBr3
Ca2Br
78g of potassium combines with 160g of bromine. What is the simplest formula of potassium bromide? (Relative atomic masses: K = 39 and Br = 80) [emp-14]
KBr2
KBr
K2Br
KBr4
48g of magnesium combines with 320g of bromine, what is the formula of magnesium bromide? (Relative atomic masses: Mg = 24 and Br = 80) [emp-15]
MgBr
Mg2Br
MgBr2
MgBr3
160g of calcium combines with 64g of oxygen, what is the formula of calcium oxide? (Relative atomic masses: Ca = 40 and O = 16) [emp-16]
CaO2
Ca2O
Ca4O4
CaO
What is the formula of the phosphorus oxide formed when 62g of phosphorus combines with 80g of oxygen? (Relative atomic masses: P = 31 and O = 16) [emp-17]
P2O5
PO
P2O
PO5
72g of magnesium combines with 48g of oxygen, what is the formula of magnesium oxide? (Relative atomic masses: Mg = 24 and O = 16) [emp-18]
MgO2
MgO
Mg3O3
Mg2O
46g of sodium combined with 16g of oxygen. What is the formula of the sodium oxide formed? (Relative atomic masses: Na = 23 and O = 16) [emp-19]
NaO
NaO2
Na2O
Na2O2
78g of potassium combined with 32g of sulphur. What is the formula of potassium sulphide formed? (Relative atomic masses: K = 39 and S = 32) [emp-20]
KS
KS2
K2S2
K2S
56g of iron combined with 240g of bromine. What is the formula of the iron bromide formed? (Relative atomic masses: Fe = 56 and Br = 80) [emp-21]
FeBr3
FeBr2
Fe3Br
FeBr
If 20.9g of bismuth combines with 24.0g of bromine, what is the formula of the bismuth bromide formed? (Relative atomic masses: Bi = 209 and Br = 80) [emp-22]
BiBr
BiBr3
Bi3Br
BiBr2
If 54g of aluminium combines with 96g of sulphur, what is the formula of aluminium sulphide? (Relative atomic masses: Al = 27 and S = 32) [emp-23]
AlS
AlS3
Al2S3
Al2S
When 117g of potassium combines with 31g of phosphorus, what is the formula of the potassium phosphide formed? (Relative atomic masses: K = 39 and P = 31) [emp-24]
KP
KP3
KP2
K3P
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If 54g of aluminium combines with oxygen to form 102g of aluminium oxide, what is the formula of aluminium oxide? (Relative atomic masses: Al = 27 and O = 16) [emp-1]